Field
The present disclosure generally relates to an optical mode converter for an optical interface. More specifically, the present disclosure relates to an optical mode converter for an optical interface that includes an inverse taper with multiple mode regions and an increased minimum tip size.
Related Art
Silicon photonics is a promising technology that can provide large communication bandwidth, large density, low latency and low power consumption for inter-chip and intra-chip connections. In the last few years, significant progress has been made in developing low-cost components for use in inter-chip and intra-chip silicon-photonic connections, including: high-bandwidth efficient silicon modulators, low-loss optical waveguides, wavelength-division-multiplexing (WDM) components, and high-speed CMOS optical-waveguide photo-detectors. Because of technical advances in CMOS processes, many of these components are now available in a commercial CMOS foundry, which may facilitate mass-volume production and, therefore, cost-effective inter-chip and intra-chip interconnects.
In order to enable dense integration, the optical mode in a silicon optical waveguide is typically sub-micron in size. However, when the optical waveguide is used to transfer data in and out from a silicon photonics chip (i.e., to optically couple light in or out from the silicon photonics chip), it usually suffers from significant optical coupling loss because of a huge optical mode size mismatch with external devices, such as an optical fiber (which has an optical mode size of around 10 μm) or a III-V optical gain chip (which has an optical mode size of around 1-3 μm). This severe optical coupling loss adversely affects the energy efficiency of the overall optical link and system.
In order to address this problem, different types of optical mode converters have been implemented in a submicron silicon-on-insulator (SOI) platform. For example, a surface-normal grating coupler has been widely adopted for versatile optical input/output (I/O) because of its excellent CMOS compatibility and wafer-level processing. However, surface-normal grating couplers also exhibit drawbacks, such as a relatively narrow bandwidth (typically 30 nm), polarization dependence, and strong back-reflection. For a specific application such as hybrid silicon-III-V laser integration, an optical interface with broadband, ultra-low coupling loss and low back-reflection is often very critical. Consequently, a surface-normal grating coupler is usually not suitable for these applications.
An optical mode converter is another widely used component in an optical interface between a silicon photonics chip and an optical fiber or other external device that has an expanded mode size. Typically, an optical mode converter on an SOI platform includes a silicon inverse taper and a dielectric (or a polymer) overcladding-type optical waveguide. A silicon inverse taper in conjunction with an overcladding dielectric waveguide has been successfully implemented to convert a submicron optical waveguide mode into an optical mode of a few microns with a very low loss.
In particular, an optical mode converter typically has very low loss because of the optical mode transition enabled by the adiabatic inverse taper with a tip size of approximately 60-80 nm. However, the widely used krypton-fluoride deep-ultraviolet lithography in foundries is currently targeting feature sizes or critical dimensions of around 100-250 nm and argon-fluoride deep-ultraviolet lithography is currently targeting feature sizes or critical dimensions of around 100 nm. Therefore, a 60-80 nm silicon inverse-taper tip size is clearly a challenge for processing using standard CMOS processes. Indeed, most of the existing optical mode converters were fabricated using low-throughput electron-beam lithography. While the current most-advanced CMOS process line with argon-fluoride immersion lithography is capable of feature sizes or critical dimensions of around 20-45 nm, such processing is not widely available and, therefore, may significantly increase the cost of optical mode converters and silicon photonics chips that include the optical mode converters.
Hence, what is needed is an optical mode converter that does not suffer from the above-described problems.